Certain laser applications, such as a Raman laser with controllable suppression of parasitic radiation, disclosed and claimed by E. V. George in co-pending U.S. patent application Ser. No. 473,178 filed Mar. 8, 1983 and assigned to the same assignee, require generation of two predetermined, distinct laser radiation frequencies and propagation of the two beams containing these emanations along substantially the same optical path within the remainder of the laser system.
Dichroic devices such as diffraction gratings can be used to combine distinct frequencies converging on the dichroic device from different directions to produce an output directed along a single optical path; but such devices are often not very efficient. A "bare", partially transmissive mirror, oriented at a predetermined angle to combine two optical beams (transmitted and reflected) converging from different directions can also be employed. However, once again a substantial fraction (.ltoreq.50%) of the energy of at least one of these beams is lost at each pass, and this can be unacceptable in a multipass system.
It would be desirable to provide a method and apparatus for propagating two distinct laser beams along substantially the same optical path while minimizing loss of energy in either beam. It would also be desirable to provide the above referenced method and apparatus where the two distinct frequencies are closely spaced.